1. Field of the Invention
The present invention relates generally to nuclear reactors. More specifically, the present invention relates to a technique which enables the repair and/or reinforcement of shrouds which have developed cracks or the like types of faults.
2. Brief Description of the Related Art
Nuclear reactors such as a boiling water type (BWR) 10 shown in FIG. 1, enclose a plurality of spaced fuel rods, generally denoted by the numeral 12, within a shroud 14 which is supported within the reactor pressure vessel (RPV) 16. The shroud 12 locates on the core support plate 20 and the top guide 32. It also supports a separator arrangement 22 and maintains a volume of water over the fuel 12. In this arrangement, a plurality of control rods 18 are disposed below the fuel rods 12 and are arranged to move up through the core support plate 20 into position to control the combustion of the nuclear fuel. The separator arrangement 22 is located on top of the shroud 22, while a dryer 24 is arranged at a still higher level and essentially opposite a main steam line 26. A pressure vessel head 28 is secured to the upper flange of the reactor pressure vessel in a manner which hermetically seals off the top of the RPV 16.
As shown in FIG. 2, the shroud 14 is constructed of a number of cylindrical and/or annular sections which are welded together. In the shroud shown in FIG. 2, only seven (H1 to H7) of the welds, which form essential parts of the shroud structure, are shown. However, it is possible that, due to intergrannular stress corrosion cracking (IGSCC) and other metallurgical phenomenon, cracks can develop in the shroud, particularly at the sites of the welds, and lead to a situation wherein portions of the shroud can separate in an undesirable manner which may interfere with the proper operation of the reactor.
Repair of such types of cracks is of course difficult and usually requires the fuel (fuel rods 12) to be offloaded and the repair carried out with the fuel stored in a separate holding pool. However, as will De appreciated, in order for the fuel rods 12 to be released, lifted out of the reactor vessel and immersed in a holding tank, all of the apparatus which is disposed in the reactor above the fuel rods must first be removed and placed in holding tanks. Accordingly, the number of operations which must be carried out before the actual repair work can actually begin is substantial and the length of the outage of the reactor is prolonged considerably. Even in the case of relatively minor cracks, the above-mentioned troublesome removal of the dryer 24, the shroud head and separator arrangement 22, and the fuel rods 12 must be carried out before repair operations can be initiated, irrespective of the fact that the repair operation itself may be relatively simple and quickly carried out.